We report the covalent layer-by-layer construction of polyelectrolyte multilayer (PEM) films by using an efficient electrochemically triggered Sharpless click reaction. The click reaction is catalyzed by Cu(I) which is generated in situ from Cu(II) (originating from the dissolution of CuSO(4)) at the electrode constituting the substrate of the film. The film buildup can be controlled by the application of a mild potential inducing the reduction of Cu(II) to Cu(I) in the absence of any reducing agent or any ligand. The experiments were carried out in an electrochemical quartz crystal microbalance cell which allows both to apply a controlled potential on a gold electrode and to follow the mass deposited on the electrode through the quartz crystal microbalance. Poly(acrylic acid) (PAA) modified with either alkyne (PAA(Alk)) or azide (PAA(Az)) functions grafted onto the PAA backbone through ethylene glycol arms were used to build the PEM films. Construction takes place on gold electrodes whose potentials are more negative than a critical value, which lies between -70 and -150 mV vs Ag/AgCl (KCl sat.) reference electrode. The film thickness increment per bilayer appears independent of the applied voltage as long as it is more negative than the critical potential, but it depends upon Cu(II) and polyelectrolyte concentrations in solution and upon the reduction time of Cu(II) during each deposition step. An increase of any of these latter parameters leads to an increase of the mass deposited per layer. For given buildup conditions, the construction levels off after a given number of deposition steps which increases with the Cu(II) concentration and/or the Cu(II) reduction time. A model based on the diffusion of Cu(II) and Cu(I) ions through the film and the dynamics of the polyelectrolyte anchoring on the film, during the reduction period of Cu(II), is proposed to explain the major buildup features.
A simple method for the nanotexturing and orientation of regioregular poly(3-hexylthiophene) (P3HT) thin films has been developed. Epitaxial growth of P3HT on the surface of an aromatic salt (potassium 4-bromobenzoate) (K-BrBz) leads to highly oriented and nanotextured P3HT films which consist of a regular network of interconnected semicrystalline domains oriented along two preferential in-plane directions. The overall crystallinity and the level of in-plane orientation of the P3HT films are controlled by the temperature of isothermal crystallization (T iso ). Well-defined electron diffraction patterns with sharp reflections obtained for T iso =180 °C indicate that the crystalline domains grow with a unique (1 0 0) P3HT contact plane on the K-BrBz substrate with the P3HT chains oriented along the [0 (2 1] K-BrBz directions of the substrate. During the annealing of the polymer film, the surface of the aromatic salt undergoes a topographic reconstruction resulting in a regular nanostructured "hill and valley" topography that templates and orients the growth of P3HT. Preferred orientation of P3HT crystalline domains occurs at step edges of the substrate and is favored by the matching between the layer period of P3HT and the terrace height of the K-BrBz substrate.
The morphology and structure of the self-assembled surfactant aggregates at the solid-liquid interface remain controversial. For the well studied system of cationic cetyltrimethylammonium bromide (C16TAB) adsorbed onto the opposite negatively charged atomically smooth mica surface, a variety of surface aggregates have been previously reported: AFM imaging pointing to cylinders and surface micelles as opposed to mono/bilayered-like structures revealed by neutron and X-ray reflectometry, NMR, spectroscopic techniques, and numerical simulations. In order to reconcile with the latter results, we revisit the morphometry of the C16TAB-coated mica surfaces using the recent Peak Force Tapping (PFT-AFM) mode that allows fragile structures to be imaged with the lowest possible applied force. The evolution of the structural organization at the mica-water interface is investigated above the Krafft boundary over a wide concentration range (from 1/1000 cmc to 2 cmc) after long-equilibration times to insure thermodynamic equilibrium. A complex but fairly complete picture has emerged: At very low concentrations the C16TAB surfactants adsorb as isolated molecules before forming small clusters. Above 1/140 cmc, monolayeredlike stripes are formed. As the concentration is increased, a connected network of these patches covers progressively the mica substrate. Above 1/80 cmc, bilayered-like patches build on top of the underlying monolayer and ultimately a complete bilayer (at about half the cmc) covers the entire mica substrate. Thanks to the less invasive PFT-AFM imaging mode, our observations agree not only with the theoretical predictions and numerical simulations, but also reconcile, at last, the direct observations by means of the AFM imaging technique with the results obtained with other techniques.
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